Can you explain reduction drives to me?

[JohnnyB]

Guys,
As I am shopping for an UL and I am seeing many direct-drive and reduction-drive choices.
I'm hearing about B-Boxes - C-Boxes, etc.

If it's not too much trouble can someone give me the skinny on all this, and why I might
want or not want these features?

I'm just a Cessna guy here and trying to learn all I can before I choose a plane.
Thanks
John - Columbus Ohio

[martymayes]

many non-aviation engines develop their power at an RPM which is too fast for a propeller to operate efficiently. That is especially true with two-stroke engines. There needs to be a way to turn the engine fast but propeller slow. That's where a reduction drive comes in. If you have a fast turning engine and need a large(r) dia prop, there will have to be a propeller speed reduction unit or PSRU in there.
Yes, they come in a lot of flavors, gears, belts, etc.

[Norman Langlois]

They are also designed to load the engine properly with a given prop. Improper engine loads are a concern for sudden engine failure. Just buying an engine with a redrive not knowing ,what the range of prop size and pitch. Be aware of these and ask for specs.

[JohnnyB]

many non-aviation engines develop their power at an RPM which is too fast for a propeller to operate efficiently. That is especially true with two-stroke engines. .

This is very helpful.
So am I to understand that on the planes I see on youtube with direct-drive engines the prop needs to be smaller?
And if you go for a larger prop to gain more thrust you need to slow it down with gear reduction?
Thanks
JB

[JohnnyB]

They are also designed to load the engine properly with a given prop. Improper engine loads are a concern for sudden engine failure. Just buying an engine with a redrive not knowing ,what the range of prop size and pitch. Be aware of these and ask for specs.

Thank you,
This seems like a very important consideration as the wrong prop could put too much load on the engine and could result in damage or failure.
Is that right?
JB

[Norman Langlois]

Yes a combination that over revs is as bad as one that is over loaded. The engines that are being used have been by trial I assume . Have an optimized rpm with load . In order to maintain cylinder head temp. Know as cht. And exhaust temps known as egt . Both important for the overal longevity of these mostly 2 Stroke engines. A direct drive such as the half VW has a prop size more relevant to the tip speed. Typically most operations target around 2600 to 3000 rpms at the output 2 stroke engines optimize power output near 6500 rpms but can not run at this continuously. This makes a reduction nessisary . One needs to take time to set up the carb ,the prop , the pitch to optimize and maintain there rpm and temps through a proper engine loading . All of this starting with recommended components ending with those all important temps and rpms. Operating outside of manufactures recommendations is risk .